Voltage detection circuit and voltage detection method

ABSTRACT

Disclosed herein is a voltage detection circuit including: a voltage detection section; a first voltage determination section; and a second voltage determination section.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage detection circuit and avoltage detection method.

2. Description of the Related Art

In order to operate an electronic circuit in a stable manner, areset-voltage detection circuit is generally used. When a power-supplyvoltage supplied to the electronic circuit becomes equal to or lowerthan a reset voltage determined in advance, the reset-voltage detectioncircuit detects the power-supply voltage becoming equal to or lower thanthe reset voltage and resets a power-supply block employed in thereset-voltage detection circuit as a block which operates by receivingthe power-supply voltage in order to supply a power to the electroniccircuit.

FIG. 3 is an explanatory block diagram showing the existingreset-voltage detection circuit 10. As shown in the explanatory blockdiagram of FIG. 3, the existing reset-voltage detection circuit 10 fordetecting a reset voltage employs a reset IC (Integrated Circuit) 11 anda power-supply block 12.

The reset IC 11 normally supplies a signal set at a high level to a CEterminal of the power-supply block 12 from an OUT terminal of the resetIC 11. Receiving a power-supply voltage denoted by reference notationPower IN at a VIN terminal of the power supply block 12, thepower-supply block 12 is made capable of operating by the high-levelsignal received from the reset IC 11.

When a power-supply voltage Power IN becomes equal to or lower than areset voltage and the reset IC 11 detects this phenomenon through a VINterminal of the reset IC 11, the reset IC 11 supplies a signal at a lowlevel to the CE terminal of the power-supply block 12. Receiving thelow-level signal received from the reset IC 11 at the CE terminal, thepower-supply block 12 stops its operation, that is, the power-supplyblock 12 is reset.

When the power-supply block 12 is reset, the current consumed by thepower-supply block 12 decreases. Thus, a voltage detected by the resetIC 11 at the VIN terminal rises. This is because a voltage drop D0through a resistor R0 also decreases as well. When the voltage detectedby the reset IC 11 at the VIN terminal rises to a level higher than thereset voltage, the low-level signal supplied by the reset IC 11 to thepower-supply block 12 changes to the signal set at the high level. Whenthe low-level signal supplied by the reset IC 11 to the power-supplyblock 12 changes to the signal set at the high level, the power-supplyblock 12 resumes its operation.

Let us think a case in which, at a point of time the power-supplyvoltage Power IN becomes equal to 2 V detected by the reset IC 11through the VIN terminal, the reset IC 11 supplies the signal at a lowlevel to a CE terminal of the power-supply block 12. Since thepower-supply voltage Power IN is an analog voltage, the power-supplyvoltage Power IN may repeatedly vary its level back and forth over asmall range such as a range of about 2 V+0.01 V. In this case, thesignal output by the reset IC 11 to the power-supply block 12 alsorepeatedly changes back and forth over a small range.

In order to solve this problem, the reset IC 11 is normally providedwith a hysteresis of about several tens of mV so that the signal outputby the reset IC 11 to the power-supply block 12 does not repeatedlychange back and forth over a small range even if the power-supplyvoltage Power IN repeatedly varies its level back and forth over a smallrange.

Depending on the configuration of the reset-voltage detection circuit10, however, the power-supply voltage Power IN may repeatedly changeback and forth over a range greater than several tens of mV. Forexample, the power-supply voltage Power IN may repeatedly change backand forth over a range of 2 V. In this case, by merely providing thereset IC 11 with a hysteresis, it may be impossible to cope with suchlarge voltage changes. In order to solve this problem, the existingreset-voltage detection circuit 10 is provided with a capacitor forsetting a large time constant for avoiding consecutive ON and OFFoperations (or oscillations) of the power-supply block 12.

SUMMARY OF THE INVENTION

However, the capacitor provided in the existing reset-voltage detectioncircuit 10 to serve as a capacitor for setting a large time constantdoes not solve the basic cause of the large voltage changes of thepower-supply voltage Power IN. Thus, there has been raised a problemthat the consecutive ON and OFF operations (or oscillations) of thepower-supply block 12 occur, depending on the conditions. In addition,the capacitor provided for the existing reset-voltage detection circuit10 increases the size of the reset-voltage detection circuit 10 andraises the cost of the reset-voltage detection circuit 10.

By the way, such a hysteresis can be set by making use of amicrocomputer. In this case, however, the use of the microcomputerraises a problem of an increased power consumption. In particular, it isdesirable to provide the reset-voltage detection circuit 10 with assimple a configuration as possible in the case of a reset-voltagedetection circuit 10 designed for an apparatus desired to consume littlepower. That is to say, in a system having large changes in power-supplyvoltage, it is necessary to assure a large hysteresis and simplify theconfiguration of the reset-voltage detection circuit.

Addressing the problems described above, inventors of the presentinvention have proposed a new and improved voltage detection circuitallowing a hysteresis capable of coping with large changes inpower-supply voltage to be set at any arbitrary value and proposed a newand improved voltage detection method to be adopted in the voltagedetection circuit.

In order to solve the problems described above, in accordance with amode of the present invention, there is provided a voltage detectioncircuit employing: a voltage detection section configured to output afirst signal when detecting a downward transition of a supplied voltageto a relatively low level equal to or lower than a first voltage oroutput a second signal when detecting the supplied voltage in a state ofbeing higher than the first voltage and when detecting an upwardtransition of the supplied voltage to a relatively high level higherthan a second voltage higher than the first voltage after the downwardtransition of the supplied voltage to the relatively low level equal toor lower than the first voltage; a first voltage determination sectionconfigured to put a first transistor in a turned-on state by making useof the second signal output by the voltage detection section so as toput a second transistor in a turned-off state in order to set a fractionof the supplied voltage at a first value so as to generate a firstfractional voltage, which is to be actually compared with a voltagedetermined in advance in order to determine whether or not the suppliedvoltage itself is equal to or lower than the first voltage, inaccordance with a first voltage ratio; and a second voltagedetermination section configured to put the first transistor in aturned-off state by making use of the first signal output by the voltagedetection section so as to put the second transistor in a turned-onstate in order to set a fraction of the supplied voltage at a secondvalue smaller than the first value so as to generate a second fractionalvoltage, which is to be actually compared with the voltage determined inadvance in order to determine whether or not the supplied voltage itselfis higher than the second voltage, in accordance with a second voltageratio.

In accordance with the configuration described above, the voltagedetection section outputs a first signal when detecting a downwardtransition of a supplied voltage to a relatively low level equal to orlower than a first voltage or outputting a second signal when detectingthe supplied voltage in a state of being higher than the first voltageand when detecting an upward transition of the supplied voltage to arelatively high level higher than a second voltage higher than the firstvoltage after the downward transition of the supplied voltage to therelatively low level equal to or lower than the first voltage. The firstvoltage determination section sets the first voltage in accordance witha first voltage ratio whereas the second voltage determination sectionsets the second voltage in accordance with a second voltage ratio. As aresult, a difference between the first voltage determined by the firstvoltage determination section and the second voltage determined by thesecond voltage determination section can be set as a hysteresis of thevoltage detection circuit.

The first voltage determination section may include a first resistor anda second resistor which are connected to each other in series whereasthe second voltage determination section may include a third resistorconnected in parallel to the second resistor.

In order to solve the problems described above, in accordance withanother mode of the present invention, there is provided a voltagedetection method including the steps of: determining whether or not asupplied voltage is higher than a first voltage set in accordance with afirst voltage ratio; outputting a first signal if the supplied voltageis determined to have become equal to or lower than the first voltage atthe first-voltage detection step; changing a comparison voltage to becompared with the supplied voltage from the first voltage to a secondvoltage higher than the first voltage when a first transistor is put ina turned-off state by the first signal output at the first-signaloutputting step in order to put a second transistor in a turned-onstate; determining whether or not the supplied voltage is higher thanthe second voltage set in accordance with a second voltage ratio;outputting a second signal if the supplied voltage is determined to havebecome higher than the second voltage at the second-voltage detectionstep; and changing the comparison voltage from the second voltage to thefirst voltage when the first transistor is put in a turned-on state bythe second signal output at the second-signal outputting step in orderto put the second transistor in a turned-off state.

As described above, in accordance with the embodiment of the presentinvention, it is possible to present a new and improved voltagedetection circuit allowing a hysteresis to be set at any arbitrary valueby making use of a simple configuration and present a new and improvedvoltage detection method to be adopted in the voltage detection circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the configuration of areset-voltage detection circuit according to an embodiment of thepresent invention;

FIG. 2 is an explanatory timing diagram showing timing charts of signalsappearing during operations carried out by the reset-voltage detectioncircuit according to the embodiment of the present invention; and

FIG. 3 is an explanatory block diagram showing the existingreset-voltage detection circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention is explained in detailby referring to diagrams as follows. It is to be noted thatconfiguration elements described in this invention specification andshown in the diagrams as elements having essentially functions identicalwith each other are denoted by the same reference numeral or the samereference notation and each of such configuration elements is explainedonce in order to avoid duplications of descriptions.

First of all, a reset-voltage detection circuit 100 according to theembodiment of the present invention is explained. FIG. 1 is anexplanatory diagram showing the configuration of the reset-voltagedetection circuit 100 according to the embodiment of the presentinvention. The configuration of the reset-voltage detection circuit 100according to the embodiment of the present invention is explained byreferring to the explanatory diagram of FIG. 1 as follows.

As shown in the explanatory diagram of FIG. 1, the reset-voltagedetection circuit 100 according to the embodiment of the presentinvention employs a reset IC 110, a power-supply block 120, resistorsR1, R2, R3 and R4 as well as N-channel transistors Q1 and Q2.

The reset IC 110 detects the magnitude of a power-supply voltage PowerIN at a VIN terminal of the reset IC 110 and outputs a signal from anOUT terminal of the reset IC 110 to a CE terminal of the power-supplyblock 120. Depending on the detected magnitude of the power-supplyvoltage Power IN, the signal output by the reset IC 110 can be set at ahigh or low level. As a standalone unit, the reset IC 110 of thisembodiment has a hysteresis of about 50 mV.

If the power-supply voltage Power IN is higher than a second leveldetermined in advance, the reset IC 110 outputs a signal set at a highlevel from its OUT terminal to the CE terminal of the power-supply block120. If the power-supply voltage Power IN is equal to or lower than afirst level determined in advance, on the other hand, the reset IC 110outputs a signal set at a low level from its OUT terminal to the CEterminal of the power-supply block 120.

In this embodiment, if a voltage supplied to the VIN terminal of thereset IC 110 is equal to or lower than a predetermined voltage of 1.8 V,the reset IC 110 outputs a signal set at a low level from its OUTterminal to the CE terminal of the power-supply block 120.

The power-supply block 120 receives the power-supply voltage Power INand supplies a power to a variety of circuits provided at a stagefollowing the power-supply block 120. The circuits at the stagefollowing the power-supply block 120 are not shown in the diagram ofFIG. 1 though. The power-supply block 120 receives the power-supplyvoltage Power IN at a VIN terminal of the power-supply block 120.

The power-supply block 120 is put in an operating state in accordancewith a high-level signal supplied by the reset IC 110 to the CE terminalof the power-supply block 120. The power-supply block 120 stops theoperating state thereof in accordance with a low-level signal suppliedby the reset IC 110 to the CE terminal of the power-supply block 120.That is to say, the power-supply block 120 stops the operating statethereof when the level of the power-supply voltage Power IN becomesequal to or lower than the first level determined in advance. As thelevel of the power-supply voltage Power IN becomes equal to or higherthan the second level determined in advance, the reset IC 110 outputs asignal set at a high level from the OUT terminal of the reset IC 110 tothe CE terminal of the power-supply block 120 in order to resume theoperation carried out by the power-supply block 120.

The first N-channel transistor Q1 is put in a turned-off state when thereset IC 110 outputs a signal set at a low level from the OUT terminalof the reset IC 110. With the first N-channel transistor Q1 put in aturned-off state, the second N-channel transistor Q2 is put in aturned-on state.

The resistors R1 and R2 determine the level of a voltage supplied to theVIN terminal of the reset IC 110 as a voltage to be compared by thereset IC 110 with a voltage determined in advance. In other words, theratio of the resistance of the resistor R1 to the resistance of theresistor R2 can be set at any arbitrary value which determines arelatively large fraction of the power-supply voltage Power IN to beactually compared with the so-called first voltage determined inadvance. By comparing the relatively large fraction of the power-supplyvoltage Power IN with the voltage determined in advance, it is possibleto determine whether or not the power-supply voltage Power IN is equalto or lower than the first voltage which is a voltage determined inadvance at a relatively low level.

In this embodiment, the resistance of the resistor R1 is set at 10kilo-ohms whereas the resistance of the resistor R2 is set at 3.3kilo-ohms. Let reference notation V denote the electric potential of thepower-supply voltage Power IN whereas reference notation Vo denote anelectric potential supplied to the VIN terminal of the reset IC 110 inthis embodiment. Thus, the electric potential Vo can be expressed interms of the electric potential V of the power-supply voltage Power IN,the resistance of the resistor R1 and the resistance of the resistor R2in accordance with Eq. 1 as follows.

$\begin{matrix}\begin{matrix}{V_{0} = {\frac{3.3}{10 + 3.3} \times V}} \\{= {\frac{3.3}{13.3} \times V}}\end{matrix} & (1)\end{matrix}$

Accordingly, when the electric potential V of the power-supply voltagePower IN becomes equal to or lower than 7.2 V, the electric potential Vosupplied to the VIN terminal of the reset IC 110 becomes equal to orlower than 1.79 V≈1.8 V so that, at a point of time the electricpotential V of the power-supply voltage Power IN becomes equal to orlower than 7.2 V, the reset IC 110 is put in an operating stateoutputting a signal set at a low level from the OUT terminal of thereset IC 110 to the CE terminal of the power-supply block 120.

The resistor R3 is a resistor through which a current is flowing to thefirst N-channel transistor Q1 whereas the resistor R4 is a resistorthrough which a current is flowing to the second N-channel transistorQ2. The ratio of the resistance of the resistor R1 to the combinedresistance of the resistors R2 and R4 connected in parallel to eachother can be set at any arbitrary value which determines a relativelysmall fraction of the power-supply voltage Power IN to be actuallycompared with the so-called second voltage determined in advance. Bycomparing the relatively small fraction of the power-supply voltagePower IN with the voltage determined in advance, it is possible todetermine whether or not the power-supply voltage Power IN is higherthan the second voltage which is a voltage determined in advance at alevel higher than the level of the first voltage mentioned before.

That is to say, while the reset IC 110 is outputting a signal set at thelow level, the resistors R1, R2 and R4 determining the relatively smallfraction of the power-supply voltage Power IN also indirectly set thesecond voltage serving as a recovery voltage for restoring a signaloutput by the reset IC 110 at a high level from the signal set at thelow level. The ratio of the resistance of the resistor R1 to thecombined resistance of the resistors R2 and R4 connected in parallel toeach other can be set at any arbitrary value selected in accordance withthe resistances of the resistors R1, R2 and R4.

In this embodiment, the resistance of the resistor R4 is set at 10kilo-ohms. With the resistance of the resistor R4 set at 10 kilo-ohms,the resistance of the resistor R1 set at 10 kilo-ohms and the resistanceof the resistor R2 set at 3.3 kilo-ohms, the electric potential Vosupplied to the VIN terminal of the reset IC 110 can be expressed interms of the electric potential V of the power-supply voltage Power INas well as the resistances of the resistors R1, R2 and R4 in accordancewith Eq. 2 as follows.

$\begin{matrix}\begin{matrix}{V_{0} = {\frac{\frac{3.3 \times 10}{3.3 + 10}}{10 + \frac{3.3 \times 10}{3.3 + 10}} \times V}} \\{= {\frac{33}{166} \times V}}\end{matrix} & (2)\end{matrix}$

Accordingly, when the electric potential V of the power-supply voltagePower IN rises to 9.0 V, the electric potential Vo supplied to the VINterminal of the reset IC 110 also rises to 1.79 V≈1.8 V, at which theaforementioned voltage determined in advance is set, as well so that, ata point of time the electric potential V of the power-supply voltagePower IN rises to 9.0 V, the reset IC 110 is put in an operating stateoutputting a signal set at a high level from the OUT terminal of thereset IC 110 to the CE terminal of the power-supply block 120.

The configuration of the reset-voltage detection circuit 100 accordingto the embodiment of the present invention has been described so far byreferring to the diagram of FIG. 1. Next, the operation of thereset-voltage detection circuit 100 according to the embodiment of thepresent invention is explained as follows.

FIG. 2 is an explanatory timing diagram showing timing charts of signalsappearing during operations carried out by the reset-voltage detectioncircuit 100 according to the embodiment of the present invention.

When the power-supply voltage Power IN is higher than 7.2 V, that is,when the electric potential Vo supplied to the VIN terminal of the resetIC 110 is higher than 1.8 V, the reset IC 110 outputs the second signalset at a high level from the OUT terminal of the reset IC 110 to the CEterminal of the power-supply block 120 during period (1) shown in thetiming diagram of FIG. 2.

Since the reset IC 110 is outputting the second signal set at a highlevel from the OUT terminal of the reset IC 110 to the CE terminal ofthe power-supply block 120 during period (1), the first N-channeltransistor Q1 is put in a turned-on state. With the first N-channeltransistor Q1 put in a turned-on state, no current is flowing to thebase of the second N-channel transistor Q2. Thus, the second N-channeltransistor Q2 is put in a turned-on state. As a result, no current isflowing through the second N-channel transistor Q2. Accordingly, nocurrent is flowing through the resistor R4. That is to say, the resistorR4 is put in a state of being electrically disconnected from the resetIC 110. In this state, the electric potential Vo supplied to the VINterminal of the reset IC 110 has a relatively high level because theelectric potential Vo is determined merely by the ratio of theresistance of the resistor R1 to the resistance of the resistor R2. Bycomparing the relatively high level of electric potential Vo supplied tothe VIN terminal of the reset IC 110 with the predetermined voltage of1.8 V, it is possible to determine whether or not the power-supplyvoltage Power IN itself is equal to or lower than the first voltage of7.2 V.

As the power-supply voltage Power IN becomes equal to or lower than thefirst voltage of 7.2 V during period (2) shown in the timing diagram ofFIG. 2, that is to say, as the electric potential Vo supplied to the VINterminal of the reset IC 110 at a level expressed by Eq. 1 becomes equalto or lower than the predetermined voltage of 1.8 V, the reset IC 110outputs a first signal set at a low level from the OUT terminal of thereset IC 110 to the CE terminal of the power-supply block 120.

Since the reset IC 110 is outputting the first signal set at a low levelfrom the OUT terminal of the reset IC 110 to the CE terminal of thepower-supply block 120 during period (2), the first N-channel transistorQ1 is put in a turned-off state. With the first N-channel transistor Q1put in a turned-off state, a current is flowing to the base of thesecond N-channel transistor Q2. Thus, the second N-channel transistor Q2is put in a turned-on state. As a result, a current is flowing throughthe second N-channel transistor Q2. Accordingly, a current is alsoflowing through the resistor R4 as well. That is to say, the resistor R4is put in a state of being electrically connected to the reset IC 110.

The resistor R4 is connected to the reset IC 110 in parallel to theresistor R2. In this state, the electric potential Vo supplied to theVIN terminal of the reset IC 110 has a relatively low level because theelectric potential Vo is determined by the ratio of the resistance ofthe resistor R1 to the combined resistance of the resistor R2 andresistor R4 which are connected to each other in parallel. By comparingthe relatively high level of the electric potential Vo supplied to theVIN terminal of the reset IC 110 with the predetermined voltage of 1.8V, it is possible to determine whether or not the power-supply voltagePower IN itself is higher than the second voltage of 9.0 V.

Since the power-supply voltage Power IN which is lower than the firstvoltage of 7.2 V is much lower than the second voltage of 9.0 V duringperiod (2), the electric potential Vo supplied to the VIN terminal ofthe reset IC 110 also becomes much lower than the predetermined voltageof 1.8 V. With the resistor R4 connected to the reset IC 110, in orderfor the electric potential Vo supplied to the VIN terminal of the resetIC 110 to become higher than the predetermined voltage of 1.8 V, it isnecessary to raise the power-supply voltage Power IN to a level higherthan the second voltage of 9 V.

Let us assume that the power-supply voltage Power IN rises to the firstvoltage of 7.2 V again after becoming lower than the first voltage of7.2 V in a transition from period (2) to period (3). Even if thepower-supply voltage Power IN rises to the first voltage of 7.2 V, theelectric potential Vo supplied to the VIN terminal of the reset IC 110is still lower than the predetermined voltage of 1.8 V because the ratioof the combined resistance of the resistor R2 and resistor R4 which areconnected to each other in parallel to the resistance of the resistor R1is relatively low. Thus, the reset IC 110 is sustaining the first signaloutput from the OUT terminal of the reset IC 110 to the CE terminal ofthe power-supply block 120 at the low level as it is during period (3).

As the power-supply voltage Power IN is restored to the level aboutequal to the second voltage of 9 V in a transition from period (3) toperiod (4), however, the electric potential Vo supplied to the VINterminal of the reset IC 110 at a level expressed by Eq. 2 is alsorestored to the predetermined voltage of 1.8 V as well. When theelectric potential Vo supplied to the VIN terminal of the reset IC 110at a level expressed by Eq. 2 is restored to the predetermined voltageof 1.8 V, the reset IC 110 is outputting a second signal set at a highlevel from the OUT terminal of the reset IC 110 to the CE terminal ofthe power-supply block 120 during period (4).

During period (4), the reset IC 110 is outputting the second signal setat a high level from the OUT terminal of the reset IC 110 to the CEterminal of the power-supply block 120 in the same way as period (1).Thus, the first N-channel transistor Q1 is put in a turned-on state bythe second signal set at a high level. With the first N-channeltransistor Q1 put in a turned-on state, no current is flowing to thebase of the second N-channel transistor Q2. Thus, the second N-channeltransistor Q2 is put in a turned-off state. As a result, no current isflowing through the second N-channel transistor Q2. Accordingly, nocurrent is flowing through the resistor R4. That is to say, the resistorR4 is again put in a state of being electrically disconnected from thereset IC 110.

When the resistor R4, which has been electrically connected to the resetIC 110 in parallel to the resistor R2 so far during periods (2) and (4)is again put in a state of being electrically disconnected from thereset IC 110, the electric potential Vo supplied to the VIN terminal ofthe reset IC 110 has a relatively high level because the electricpotential Vo is determined merely by the ratio of the resistance of theresistor R1 to the resistance of the resistor R2. Thus, the relativelyhigh level of the electric potential Vo supplied to the VIN terminal ofthe reset IC 110 further rises from the predetermined voltage of 1.8 Vdue to the increase in power-supply voltage Power IN.

Accordingly, the reset IC 110 outputs the first signal set at the lowlevel from the OUT terminal of the reset IC 110 to the CE terminal ofthe power-supply block 120 merely when the electric potential Vosupplied to the VIN terminal of the reset IC 110 becomes equal to orlower than the predetermined voltage of 1.8 V, that is, merely when thepower-supply voltage Power IN itself becomes equal to or lower than thefirst voltage of 7.2 V and sustains the first signal as it is till theelectric potential Vo supplied to the VIN terminal of the reset IC 110becomes higher than the predetermined voltage of 1.8 V, that is, tillthe power-supply voltage Power IN itself becomes higher than the secondvoltage of 9.0 V. As a result, the reset-voltage detection circuit 100shown in the diagram of FIG. 1 has a hysteresis of about 1.8 V (=9 V−7.2V), which can be proved to be equal to the predetermined voltage of 1.8V.

As described above, the reset IC 110 serving as standalone unit can havea hysteresis of about 50 mV. By adding the resistors R1, R2, R3 and R4as well as the N-channel transistors Q1 and Q2 each serving as aswitching device to the reset IC 110 in accordance with the embodimentas described above, however, the reset-voltage detection circuit 100 asa whole can be provided with a hysteresis for controlling the electricpotential Vo supplied to the VIN terminal of the reset IC 110 inaccordance with the state of a signal output from the OUT terminal ofthe reset IC 110 to the CE terminal of the power-supply block 120.

By controlling the electric potential Vo supplied to the VIN terminal ofthe reset IC 110 through the use of the resistors R1, R2, R3 and R4 aswell as the N-channel transistors Q1 and Q2 each serving as a switchingdevice in accordance with the embodiment as described above, it ispossible to provide the reset-voltage detection circuit 100 with ahysteresis larger than the hysteresis of the reset IC 110 serving asstandalone unit while keeping the configuration of the reset-voltagedetection circuit 100 simple. In addition, the hysteresis given to thereset-voltage detection circuit 100 can be set at any arbitrary valuewhich is determined by the resistances of the resistors R1, R2 and R4.

As a result, in the case of an application in which the power-supplyvoltage Power IN changes much, the hysteresis given to the reset-voltagedetection circuit 100 can be set at a large value useable for copingwith the large change in power-supply voltage Power IN by making use ofa simple configuration so that the signal output from the OUT terminalof the reset IC 110 to the CE terminal of the power-supply block 120does not change every time the power-supply voltage Power IN crosses athreshold level.

The above description explains operations carried out by thereset-voltage detection circuit 100. As described above, in accordancewith the embodiment of the present invention, by adding the resistorsR1, R2, R3 and R4 as well as the N-channel transistors Q1 and Q2 eachserving as a switching device to the reset IC 110 in a simpleconfiguration in accordance with the embodiment as described above, itis possible to provide the reset-voltage detection circuit 100 with ahysteresis larger than the hysteresis of the reset IC 110 serving asstandalone unit.

The preferred embodiment of the present invention has been describedabove by referring to diagrams. It is needless to say, however, thatimplementations of the present invention are by no means limited to thepreferred embodiment. It is obvious that, inspired by the preferredembodiment, a person skilled in the art is capable of coming up with avariety of modified versions or changed configurations within rangesdefined by claims appended to this invention specification. However,such modified versions and changed configurations are regarded asversions and changes naturally falling within the technological range ofthe present invention.

For example, each of the resistors R1, R2 and R4 employed in thereset-voltage detection circuit 100 according to the embodiment has aconstant resistance. However, implementations of the present inventionare by no means limited to such a scheme according to the preferredembodiment. That is to say, at least, one of the resistors R1, R2 and R4may have a variable resistance. By employing at least one of theresistors R1, R2 and R4 as a resistor having a variable resistance, thehysteresis can be set at any arbitrary value even after thereset-voltage detection circuit has been configured.

In addition, it should be understood by those skilled in the art that avariety of modifications, combinations, sub-combinations and alterationsmay occur, depending on design requirements and other factors insofar asthey are within the scope of the appended claims or the equivalentsthereof.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-133482 filedin the Japan Patent Office on May 21, 2008, the entire content of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factor in so far as they arewithin the scope of the appended claims or the equivalents thereof.

1. A voltage detection circuit comprising: a voltage detection sectionconfigured to output a first signal when detecting a downward transitionof a supplied voltage to a relatively low level equal to or lower than afirst voltage or output a second signal when detecting said suppliedvoltage in a state of being higher than said first voltage and whendetecting an upward transition of said supplied voltage to a relativelyhigh level higher than a second voltage higher than said first voltageafter said downward transition of said supplied voltage to saidrelatively low level equal to or lower than said first voltage; a firstvoltage determination section configured to put a first transistor in aturned-on state by making use of said second signal output by saidvoltage detection section so as to put a second transistor in aturned-off state in order to set a fraction of said supplied voltage ata first value so as to generate a first fractional voltage, which is tobe actually compared with a voltage determined in advance in order todetermine whether or not said supplied voltage itself is equal to orlower than said first voltage, in accordance with a first voltage ratio;and a second voltage determination section configured to put said firsttransistor in a turned-off state by making use of said first signaloutput by said voltage detection section so as to put said secondtransistor in a turned-on state in order to set a fraction of saidsupplied voltage at a second value smaller than said first value so asto generate a second fractional voltage, which is to be actuallycompared with said voltage determined in advance in order to determinewhether or not said supplied voltage itself is higher than said secondvoltage, in accordance with a second voltage ratio.
 2. The voltagedetection circuit according to claim 1 wherein said first voltagedetermination section includes a first resistor and a second resistorwhich are connected to each other in series, and said second voltagedetermination section includes a third resistor connected in parallel tosaid second resistor.
 3. A voltage detection method comprising the stepsof: determining whether or not a supplied voltage is higher than a firstvoltage set in accordance with a first voltage ratio; outputting a firstsignal if said supplied voltage is determined to have become equal to orlower than said first voltage at said first-voltage detection step;changing a comparison voltage to be compared with said supplied voltagefrom said first voltage to a second voltage higher than said firstvoltage when a first transistor is put in a turned-off state by saidfirst signal output at said first-signal outputting step in order to puta second transistor in a turned-on state; determining whether or notsaid supplied voltage is higher than said second voltage set inaccordance with a second voltage ratio; outputting a second signal ifsaid supplied voltage is determined to have become higher than saidsecond voltage at said second-voltage detection step; and changing saidcomparison voltage from said second voltage to said first voltage whensaid first transistor is put in a turned-on state by said second signaloutput at said second-signal outputting step in order to put said secondtransistor in a turned-off state.
 4. A voltage detection circuitcomprising: voltage detection means for outputting a first signal whendetecting a downward transition of a supplied voltage to a relativelylow level equal to or lower than a first voltage or outputting a secondsignal when detecting said supplied voltage in a state of being higherthan said first voltage and when detecting an upward transition of saidsupplied voltage to a relatively high level higher than a second voltagehigher than said first voltage after said downward transition of saidsupplied voltage to said relatively low level equal to or lower thansaid first voltage; first voltage determination means for putting afirst transistor in a turned-on state by making use of said secondsignal output by said voltage detection means so as to put a secondtransistor in a turned-off state in order to set a fraction of saidsupplied voltage at a first value so as to generate a first fractionalvoltage, which is to be actually compared with a voltage determined inadvance in order to determine whether or not said supplied voltageitself is equal to or lower than said first voltage, in accordance witha first voltage ratio; and second voltage determination means forputting said first transistor in a turned-off state by making use ofsaid first signal output by said voltage detection means so as to putsaid second transistor in a turned-on state in order to set a fractionof said supplied voltage at a second value smaller than said first valueso as to generate a second fractional voltage, which is to be actuallycompared with said voltage determined in advance in order to determinewhether or not said supplied voltage itself is higher than said secondvoltage, in accordance with a second voltage ratio.